OS	Arch	Compiler	MPI	Status
Ubuntu 24.04	x86_64	gfortran 14	OpenMPI / MPICH
Ubuntu 24.04	x86_64	ifort 2024 / ifx 2025	Intel MPI
macOS 15	ARM64	gfortran 14	OpenMPI

nekStab is a toolbox for global stability and bifurcation analysis using the spectral element solver Nek5000. Released under BSD-3-Clause license.

• Wavemaker — structural sensitivity to feedback
• Energy budget — production, dissipation, transport decomposition (steady + Floquet)
• Base flow sensitivity — response to mean flow modifications
• Steady force sensitivity — optimal placement for passive control
• Mode animation — reconstruct eigenmode dynamics over one period

• OTD modes — real-time Lyapunov vectors for chaotic flows
• Linearized DNS — perturbation evolution around base flows
• Scalar transport — temperature and passive scalar stability
• Sponge zones — non-reflecting boundaries for open flows
• Vortex identification — λ₂, Q-criterion, Ω-criterion output
• FFT module — FFTW3/MKL integration for spectral analysis with Nek5000

• Matrix-free: No Jacobian storage — scales to millions of DoFs
• Krylov-based: Time-stepper as linear operator — eigenvalues from snapshots
• Spectral accuracy: Leverages Nek5000's high-order elements
• MPI parallel: Efficient on laptops to supercomputers
• Validated: Automated test suite covering 9 solver modes across 4 geometries (cylinder, backstep, flip-flop, jet)
• FFT included: Smart linking to FFTW3 (GCC) or MKL (Intel) — no manual setup

See DOC.md for full parameter reference.

Ready-to-run cases in example/:

Prerequisites

Linux (GCC)

sudo apt -y install build-essential gfortran libmpich-dev libopenblas-dev libfftw3-dev cmake m4 htop

macOS

brew install mpich gfortran fftw wget git cmake htop

Installation

git clone --depth=1 https://github.com/nekStab/nekStab.git
cd nekStab
./Nek5000setup.sh

Already have Nek5000? Skip ./Nek5000setup.sh and point NEK_SOURCE_ROOT to your existing installation (see below).

Add to your shell config (~/.bashrc or ~/.zshrc):

# nekStab path
export NEKSTAB_SOURCE_ROOT=$HOME/nekStab
export PATH=$NEKSTAB_SOURCE_ROOT/bin:$PATH

# Nek5000 path (adjust if using existing installation)
export NEK_SOURCE_ROOT=$NEKSTAB_SOURCE_ROOT/Nek5000  # ← or your existing path
export PATH=$NEK_SOURCE_ROOT/bin:$PATH

# Stack/core limits for large simulations
ulimit -s unlimited
ulimit -c unlimited

Then reload: source ~/.bashrc (or restart terminal).

Enable tab completion for case names and common mks targets/options:

source "$NEKSTAB_SOURCE_ROOT/bin/mks-completion.bash"

For zsh, load Bash completion first:

autoload -Uz bashcompinit && bashcompinit
source "$NEKSTAB_SOURCE_ROOT/bin/mks-completion.bash"

Go to a given example folder and compile the code:

cd ~/nekStab/example/cylinder/baseflow/newton
mks 1cyl

nekStab supports multiple Fortran compilers. The build script auto-detects available compilers in this order: ifx → ifort → gfortran. To force a specific compiler:

NEKSTAB_FC=ifx mks 1cyl    # Intel LLVM (recommended for Intel/AMD CPUs)
NEKSTAB_FC=ifort mks 1cyl  # Intel Classic (available on many HPC systems)
NEKSTAB_FC=gcc mks 1cyl    # GCC/gfortran

Note: Intel discontinued ifort in oneAPI 2025, but it remains available on many HPC systems. New installations should use ifx.

The Intel compilers use MKL (Math Kernel Library) which provides highly optimized BLAS/LAPACK routines with runtime CPU dispatching.

For debugging crashes or numerical issues:

mks 1cyl --debug

Tip: When a crash happens in debug mode, the output shows which subroutine and line number failed — much easier than hunting through a release build.

After successful compilation:

nekbmpi 1cyl 4       # Run case "1cyl" on 4 MPI processes
tail -f logfile      # Monitor output in real-time
killall nek5000      # Stop the simulation

Tip: Use nekbmpi 1cyl 4 & to run in background, then tail -f logfile in the same terminal.

For more information, see the Documentation.

Three equivalent ways to select operating mode:

nekStab is maintained by Ricardo Frantz.

See also: LightKrylov is a modern Fortran library providing abstract Krylov methods. neklab is the next-generation bifurcation and stability analysis toolbox for Nek5000, built on LightKrylov. Both are actively developed by Jean-Christophe Loiseau and Simon Kern.

Website: https://nekstab.github.io/

When using nekStab, please cite Frantz et al. (2023). See CITATIONS.md for BibTeX entries and additional references.